1. Technical Field
The present invention relates to a method for compensating head position of a hard disk drive, and more particularly, relates to a repeated control method for compensating repeatable following errors which occur due to an unbalanced spindle or by an eccentricity of magnetic disk.
2. Related Art
The development of hard disk drive technology as well as the rapid progress of VLSI and software technology has played an important role in developing the modern information industry for last decades. The trend of hard disk drive technology is toward miniaturization in terms of size, higher capacity, faster seek times and lower cost, and the density of data write is increased by almost ten times every ten years. This trend is expected to continue in the future.
The servo controller of hard disk drive is configured so as to operate two servo modes in accordance with the moving distance of a head. The first servo mode relates to a track seek mode in which a head accesses tracks of the disk to search for a target track and transition over the target track for reading and writing data on the target position. The second servo mode relates to a track settle mode or track following mode in which the head is accurately positioned on a data line of the target track after the head has reached the target track. In order to increase the density of data write, it is necessary to reduce track mis-registration ("TMR") which is a difference between the center of track and the physical position of head, and to perform high precision servo control of the head arm in the track-following mode.
The servo control technology can be classified as dedicated servo control and embedded servo control according to detection of a position error signal (PES). The dedicated servo control technique detects the PES signal by utilizing an independent original disk having only position information of head. Exemplary configuration is disclosed in U.S. Pat. No. 5,233,487 for Functional Measurement Of Data Head Misregistration issued to Christensen et al. By contrast, the embedded servo control technique utilizes position information written on every sector of data track. Exemplary configurations are disclosed in U.S. Pat. No. 4,217,612 for Servo System For Track Accessing And Track Following In A Disk Drive issued to Matla et al., U.S. Pat. No. 4,636,885 for Servo System For A Magnetic Disk Memory Having Spiral Tracks issued to Yamada et al., U.S. Pat. No. 5,541,784 for Bootstrap Method For Writing Servo Tracks On A Disk Drive issued to Cribbs et al., and U.S. Pat. No. 5,576,909 for Method For Positioning A Data Transducer Head In A Rotating Disk Drive Data Storage Device issued to Dierkes et at. Generally, the dedicated servo control technique has an advantage for designing a controller having a band wider than that of the embedded servo control technique as the dedicated servo method is able to detect position information continuously. However, the inconsistency of position information disk with data disk due to degradation of the disks still causes track misregistration. As a result, the embedded servo control technique is widely used recently.
During the track-following operation, conventional HDD servo controller has been implemented using a proportional integral derivative control method, known in the art as PID control, in order to compensate an off-track of head by obtaining PES to position a magnetic head in close contact with track center for data read and write. If the track center of the disk is consistent with the spindle center, there is no repeated ingredient in input following errors by using the conventional controller. However, the frequency ingredient having a base mode of disk rotation frequency of error ingredients may occur as it is impossible to consistently position the center of the disk with that of the spindle. As a result, repeatable run out errors often occur at the disk rotation frequency. Recent efforts in the art such as disclosed in U.S. Pat. No. 5,539,714 for Adaptive Runout Compensation For Miniature Disk Drives issued to Andrews, Jr. et al., and U.S. Pat. No. 5,585,976 for Digital Sector Servo Incorporating Repeatable Run Out Tracking issued to Pham, provide servo control systems which compensate for repeatable run out errors. However, it is my observation that these servo control systems are often complex, and further improvement can still be contemplated.